In the past, there have been proposed various position sensors configured to measure displacement of a target (e.g., rotational displacement, a rotation angle, or a rotation position of a rotative target). For example, there is a position sensor disclosed in a document 1 (Japanese patent laid-open publication No. 2008-292376). Hereinafter, a conventional instance disclosed in the document 1 is explained with reference to a figure.
As shown in FIG. 7, this conventional instance includes a detection coil 10 and an electrical conductor 20. The detection coil 10 is prepared by winding a conducting wire around a cylindrical core 10a made of nonmagnetic material. The electrical conductor 20 is disposed in a vicinity of an inside or an outside of the detection coil 10 to be displaced in an axial direction of the detection coil 10. The electrical conductor 20 is shaped into a cylindrical shape. Further, the conventional instance includes an oscillation circuit 30 and an oscillation period measurement circuit 40. The oscillation circuit 30 is configured to output an oscillation signal having its magnitude oscillating at a frequency corresponding to an inductance of the detection coil 10. The oscillation period measurement circuit 40 is configured to measure a period of the magnitude of the oscillation signal output from the oscillator 30, and output a signal indicative of the measured period. Moreover, the conventional instance includes a squaring circuit 50, a temperature compensation circuit 60, and a signal processing circuit 70. The squaring circuit 50 is configured to calculate a square value of the signal output from the oscillation period measurement circuit 40, and output the calculated square value. The temperature compensation circuit 50 is configured to compensate for a temperature variation of an output signal from the squaring circuit 50. The signal processing circuit 70 is configured to determine displacement of the electric conductor 20 on the basis of an output signal from the temperature compensation circuit 60.
This conventional instance measures an inductance variation of the detection coil 10 as the displacement of the electric conductor 20 moving together with the target. Thus, the conventional instance can measure the displacement of the target. The period of the oscillation signal includes a component indicative of a square root of an inductance component and a capacitance component. However, the squaring circuit 50 calculates the square value of the period of the oscillation signal and outputs the calculated square value. Therefore, the component indicative of the square root of the inductance component and the capacitance component is canceled. That is, the output signal from the squaring circuit 50 has its magnitude linearly varying with the displacement of the target. Accordingly, the conventional instance can provide the output signal which is convenient for calculation using the same.
With the above conventional instance, in the event of termination of the oscillation caused by breakage of the detection coil 10, no signal is input to the oscillation period measurement circuit 40. After that event, an output of the sensor is not updated anymore. In this situation, even if the electrical conductor 20 is displaced relative to the detection coil 10 after the oscillation is terminated, the output is not changed. Therefore, the sensor keeps outputting erroneous detection information with regard to the displacement of the target. In order to prevent occurrence of erroneous measurement of the displacement of the target, it is preferred to detect a malfunction in the event of the termination of the oscillation, and to output a malfunction detection signal. Such a technique of detecting breakage of the detection coil 10 (electromagnetic pick up) is disclosed in a document 2 (Japanese patent laid-open publication No. 2001-21383).
However, in the aforementioned conventional instances, there occurs an erroneous high frequency oscillation when inductance or capacitance sees a rapid decrease resulting from a malfunction such as layer short of the detection coil 10 and detachment of the capacitor C connected in parallel with the detection coil 10. In a situation where an oscillation at high frequency occurs, the displacement of the target is determined based on a signal having its magnitude oscillating at an abnormal high frequency. Therefore, the conventional position sensor is likely to output erroneous detection information with regard to the displacement of the target.